"Yes, I heard you telling him about it," answered Tommy. "You handed it to him straight. However, more about the marsupial lion later. I'm ready." "Then let 'er go," called Dodd, and his words were swallowed up by the sound of the hollow shell striking against the rocky bank as he launched his strange craft into the water. Tommy set one foot into the hollow of his shell, and let himself go. Instantly the shell shot forward with fearful velocity. It was all Tommy could do to balance himself, for it seemed more unstable than a canoe. Once or twice he thought he heard Dodd shouting ahead of him, but his cries were drowned in the rush of the torrent. * * * * * Suddenly a light appeared in the distance. Tommy thought it was another of the petroleum fountains, and his heart seemed to stand still. But then he gave a gasp of relief. It was a cluster of luminous fungi, ten or twelve feet tall, emitting a glow equal to that of a dozen 40-watt electric bulbs. By that infernal light Tommy could see that the stream curved sharply. It was about fifty feet in width, and the low rock roof had receded to some fifteen feet overhead. Instead of a tunnel, there was nothing on either side of them but a vast tract of marshy ground thinly coated with the red grass. As Tommy looked, he saw the shell that carried the unconscious body of Haidia strike the bank beside the phosphorescent growth. He could see the girl lying in the hollow of the shell, as pale as death, her eyes closed. Dodd was close behind. As the swirl of the current caught his shell, he turned to shout a warning to Tommy. And Tommy noticed a singular thing, of which his sense of balance had already warned him, though he had hardly given conscious thought to the matter. _The river was running up-hill!_ Of course it was, since the center of gravity was in the shell of the earth, and not in the center! But, again, the shell of the earth was under their feet! Then Tommy hit on the solution to the problem. If the river was running up-hill, that meant that they must be near the exterior of the earth. In other words, they had passed the center of gravity: they must be within a mile or so of the exit from Submundia! * * * * * Tommy was about to shout his discovery to Dodd when his shell grounded beside the two others, at the base of the clump of fungi. Huge, straight, hollow stems they were, with mushroom caps, and, like all fungi, fly-blown, for Tommy could see worms nearly a foot in length crawling in and out of the porous stalks. The stench from the growth was nauseating and overpowering, utterly sickening. "Push off and let's get out of here!" Tommy called to Dodd, who was balancing his shell against the bank, and trying to peer into Haidia's face. At that moment he caught sight of something that made his blood turn cold!